Hafnium vapor pressure, high temperature

Nisel son (394, 395) has concluded that any separation process based on differences in volatility should be carried out within the temperature range 440°-475°C. Vapor pressure measurements at the triple point only lead to a separation coefficient of 1.7 (415). On the other hand, small quantities of zirconium-free hafnium have been prepared by gas chromatographic techniques on a synthetic, highly purified graphite column at 295°C (543). 

Zirconium tetrachloride is a white crystalline solid with specific gravity 2.803.4 The solid sublimes under atmospheric pressure at 331° [log pBin. = — (26,000/4.57T) + 12.30] but melts at 437° under its own pressure, which is about 25 atm. at this temperature.18 The vapor density shows normal behavior up to 500°, at which point dissociation is suspected.16 Electron-diffraction results show tetrahedral symmetry with the Zr-Cl distance 2.33 A.17 Todd18 has recently determined the heat capacity of zirconium tetrachloride at low temperatures (extrapolated below —222°) and calculated the usual thermodynamic constants for the compound. The melting point of hafnium tetrachloride has been estimated as 432° under its own pressure, but the solid sublimes under atmospheric pressure at 317°.12 Both metal chlorides are extremely hygroscopic. They are easily soluble in water, although extensively hydrolyzed. All high-purity samples should be desiccated as well as possible. 

The thermal stability of the hexachlorotitanate compounds is relatively low as compared to that of the corresponding hafnium and zirconium compounds, as the hexachlorotitanates exhibit high decomposition pressures even at low temperatures. Accordingly, the solubility of TiCl4 vapor in alkali chloride melts under normal pressures is limited. For example, at 690 and 720°C the solubilities of TiCl4 vapor in the equimolar melt of KCl-NaCl are only 14 and 5 wt% respectively,when the applied pressure of the vapor was kept at 1 atm. 

---